Three Horizontal Flow Biofilm Reactors (HFBRs) were used to treat airstreams contaminated with low concentrations of methane (1.55%; 0.8 mol CH4/m3/h) at 10°C. The HFBRs were inoculated with a highly-enriched, methane-oxidizing biomass. The trial lasted 177 d and maximum removal rates of 47.9% were observed. Bespoke, in vitro, batch activity assays indicated that the potential for ethaneoxidation varied between zones, with highest rates generally found at zones closest to the nutrient feed inlet. The composition of the HFBR bacterial community was investigated over the trial using Terminal-Restriction Fragment Length Polymorphism (T-RFLP) gene fingerprinting, Fluorescence in-situ Hybridisation (FISH) and 13 C-CH4 Stable Isotope Probing (SIP) incubations. The biofilms comprised a diverse bacterial community, which changed over time. Type-I and Type-II methanotrophs were present – and active – in the biofilm. CH4 removal rates achieved in this trial indicate the excellent potential of HFBRs to biologically treat low concentrations of CH4 at 10°C.